This invention relates to a method for protecting plants from both disease and pests by means of genetic engineering to incorporate into the plant antagonistic agents or inhibitors for the infectious or harmful conditions which result. More specifically, plants suffer pathogenic conditions commonly known as diseases caused by virus, bacteria and fungus. Further, pests, such as various kinds of insects, cause untold damage to plants. The present invention provides a method for incorporating into the plant itself the means with which to deal with such pathogenic conditions.
Development of plant biology began in the early 1940s when experiments were being carried out to determine the biological principle causing formation of crown gall tumors. The tumor-inducing principle was shown to be a bacterial plasmid from the infective organism Agrobacterium tumefaciens. This plasmid has been characterized in exquisite biochemical detail utilizing the currently available techniques of recombinant DNA technology. The mode of operation for infection was the discovery that the bacterium elicits its response by actually inserting a small fragment of the bacterial plasmid into the plant nucleus where it becomes incorporated and functions as a plant gene. This discovery opened the door to using Agrobacterium and their plasmids as vehicles to carry foreign DNA to the plant nucleus. There are, however, limitations to the application of these techniques and they include: (1) susceptibility to infection with the Agrobacterium plasmid and (2) available tissue culture technology for regeneration of the transformed plants. These limitation have meant that, to date, there are no successful reports on genetic engineering of cereals because of the inability of Agrobacterium to infect cereal plants.
The plant genes, like all other genes, are simply strings of nucleic acid bases. The function of synthetic genes within the plant can be to produce a gene product which has its own intrinsic value or to produce an intermediate gene product, such as mRNA, that plays a regulatory role within the plant cell. Synthetic genes are most useable when the technical capability does not exist to isolate and purify genes from natural organisms. Both purified and synthetic genes may be used in conferring protection to plants against disease or pests. An example of the use of synthetic genes to confer resistance to viruses and viroids in plants comes through the use of the coevolution of many viruses with the plant hosts. Plants and animals have evolved very precise and elegant mechanisms which allow the regulated expression of their genes. Viruses by co-evolution have exploited and continue to exploit the eukaryotic cell of a plant by mimicking the general structure of the plant genes. Often, the end result of this for the plant is disease. While plants and animals are prisoners of their own evolution, so are viruses since they are dependent upon the plant and animal system of gene regulation and expression to synthesize and translate their own genes into the plant. This dependency is considered the key to control of viral disease. It has recently been found that bacteria regulate expression of some genes in a rather novel way. Under conditions where the cell would repress the biosynthesis of a particular protein, an additional level of control is exerted. This newly discovered type of gene control is called "micRNA" control and stands for "mRNA-interfering complementary RNA". This micRNA or "antisense" RNA is complementary to the 5' end of the gene and when it is produced has the ultimate effect of reducing the amount of messenger RNA (mRNA) by annealing to it, thus removing it from normal protein synthesis.
Viroids are single-stranded ribonucleic acid (RNA) of a few hundred nucleotides. They are the smallest self-replicating structures known and represent the lowest form of life. They are the causative agents of a number of plant diseases and elicit mild to lethal responses in a range of plants depending on the fine structure of the viroid and the susceptibility of the plant genotype. In the case of viroids, the logic is to inhibit the replication of the disease-causing molecule. Viroids are infective pieces of nucleic acid and do not have a protein component like viruses. Using methodology similar to viruses, we have the opportunity of blocking the nucleic acid replication (called transcription).
Many plants contain genes that confer virus resistance and, in some cases, resistance is due to a single dominant gene while, in other cases, the resistance is genetically more complex, i.e., requiring a number of genes to confer resistance. While it is presently practically impossible to identify, isolate and purify these resistance genes and, in fact, the chromosomes which carry these genes cannot even be located, utilizing the in vivo-produced antisense fragments, it may be possible to attain virus and viroid resistance by the insertion of a single synthetic gene which ultimately would produce an RNA complementary to specific regions of the viral genome and would play a role in the disruption of replication or translation of the virus. This complementary or antisense gene would be specific for a particular viral pathogen. Plants endowed with a number of these antisense genes would protect them from a variety of viral diseases, such as the symptoms observed in a viral disease of a potato.
In contrast to the use of one or more synthetic genes for protection of plants against viral disease, bacterial protection employs the known response of an insect to bacterial infection to confer resistance to bacterial disease. The pupae of Hyalophora (a type of silk moth) respond to bacterial infection by the synthesis of mRNAs which culminate in the production of about 15 to 20 new proteins. Lysozyme, the antibacterial protein found in egg white and human tears, and two other classes of antibacterial peptides, called cecropins and attacins, have been purified from these newly synthesized proteins. Lysozyme has been shown to be effective in limiting bacterial growth. When lysozyme was placed on a small paper dish in two concentrations, after an agar plate was seeded with plant pathenogenic bacteria, the lysozyme inhibitory zone was quite clear.
These proteins have a rather broad spectrum activity in that they are effective on many different types of bacteria. Thus, the insects have evolved a rather successful and novel means to fight bacterial infections. Although a traditional immunologist would think this system lacks specificity, the insect has a rather potent arsenal of at least three different bacterial proteins which may work in different ways to actively seek to destroy the bacterial pathogen. Thus, the invading bacteria is presented with a formidable challenge which would be very difficult to circumvent. While a bacterial pathogen may be naturally resistant to one, it is highly improbable that it would be resistant to all three toxins. Although determination of the exact mode of action of the protein toxins is required, they are quite prokaryote specific and appear to be benign to eukaryotic cells. Incorporation of the proteins derived from humoral response An Hyalophora are an attractive genetic system for protecting plants from bacterial disease which causes significant economic loss. As an example, the main diseases in potato are bacterial soft rot and bacterial wilt caused by Erwiria carotovora and Pseudomona solanacearum, respectively. These diseases are primarily responsible for limiting the growth of potatoes in many areas of Asia, Africa, South and Central America. The introduction of genes encoding for these antibacterial proteins into crop plants may revolutionize the protection of plants from bacterial-produced disease.
In a manner analogous to the antibacterial-protein producing insect, it has been found that some bacteria are natural repositories which contain genes encoding for proteins effective against fungi. The biochemical analysis of these compounds and the ultimate molecular characterization of genes encoding the synthesis of these natural antifungal agents could be of great importance in limiting the scope and severity of fungal disease in crop plants. It is believed that from 1845 to 1860 the fungal disease of the potato caused by Phytophthora infetans or Late Blight caused the Irish potato famine in which one million people died of starvation and one and a half million people immigrated to North America because of the decimation of the potato crop caused by this fungal pest.
Control of insect pests which cause tremendous losses of food and fiber derived from plants directly attributed to the insects and as a vector for the infection and spread of other plant pathogens requires increasing a plants tolerance to insect damage. Such increased tolerance would be of significant economic value. One method for protecting plants from insect damage is the use of natural insecticides such as a protein isolated from Bacillus thuringiensis which forms a high molecular weight crystal in the bacteria which is toxic to the larvae of a number of lepidopteral insects. However, it is believed that the insects would develop an immunity to the toxin within a few generations. Thus, other possibilities are being considered. One of the most promising is the use of an enzyme called chitinase as a natural insecticide. Chitinase is an enzyme produced in bacteria and the gene which encodes it has been isolated. An insect exoskeleton and gut are composed of chitin and it has been shown that any disruption of the chitin integument will result in the insect contracting an infection from the natural environment with death ensuing after a very short period of time. Inserting the enzyme-encoding gene for chitinase into the plant will provide a potent chitin-dissolving enzyme within the plant which will limit the extent of damage wrought by insects and secondarily limit the spread of other plant diseases which use insects as a vector.